Several lines of evidence suggest that accumulation of DNA damage coupled with defects in DNA repair play an important role in breast cancer. Studies using HPLC and GC-MS have revealed increased levels of 8-oxoG in invasive ductal breast carcinomas relative to normal breast tissue implicating oxidative damages in the etiology of breast cancer. It has been shown that 8-oxoG is repaired via the base excision repair (BER) pathway. To date, there are no reports on the removal of 8-oxoG or other oxidative DNA base lesions in breast cancer cells. Therefore, it remains to be established whether BER of oxidative lesions is altered during breast carcinogenesis. We examined the ability of nuclear extracts prepared from untreated MCF-7 and MDA-MB-468 breast cancer cells to incise Tg and 8-oxoG containing oligonucleotides. Nuclear extracts from both the MCF-7 and the MDA-MB-468 cells were proficient in the removal of both Tg and 8-oxoG. However, the Tg was removed more efficiently than the 8-oxoG lesion. We also examined the kinetics of the incision of 8-oxoG in nuclear and mitochondrial extracts of MCF-7 and MDA-MB-468 cells. Mitochondrial extracts incised 8-oxoG less efficiently than the nuclear extracts from the same cell harvest. Mitochondrial extracts from these cells had about 20% maximum incision that was attained at 2 hr whereas the nuclear extracts showed about 70% incision at 24 hr. These results provide evidence that repair of 8-oxoG may be different in nuclear and mitochondrial extracts of human breast cancer cells. Comparison of the incision of 8-oxoG by mitochonrial extracts from the breast cancer cell lines to those from wild type cells showed that mitochondrial extracts from both breast cancer cell lines were deficient in the incision of 8-oxoG compared to the wild type cells. The incision activity for MCF-7 and MDA-MB-468 was 3- and, 2.4-fold lower respectively than the wild type. These results suggest a possible defect in the repair of 8-oxoG by mitochondrial extracts from these breast cancer cells. Kinetic analyses of thymine glycol incision by nuclear and mitochondrial extracts from MCF-7 and MDA-MB-468 breast cancer cells revealed that both nuclear and mitochondrial extracts incised Tg to the same extent as depicted by overlapping kinetic graphs. These results indicate that enzymes involved in the removal of Tg may be the same in nuclear and mitochondria of human mammary cells, and that mitochondrial extracts from these breast cancer cell lines are efficient in Tg incision. These observations rule out the idea that mitochondria intrinsically have lower DNA repair capacity than nuclear extracts. Furthermore, the results affirm that the observed decreased 8-oxoG activity is specific for that lesion and restricted to mitochondria of breast cancer cells. Since the repair of Tg by the breast cancer extracts was comparable to that by wild type extracts, the data suggest that the BER sub-pathway for the repair of 8-oxoG is defective in the mitochondria of breast cancer cells. This defect was not due to a lower expression of the putative enzyme, hOgg1 as Western analyses revealed that b-hOgg1 was present in the mitochondria of both breast cancer cell lines at equal or higher amounts relative to the wild type. We conclude that mechanisms of 8-oxoG repair in breast mitochondria may be crucial in the development of breast cancer.